It is a well known practice in certain engineering design developments to devote substantial efforts toward eliminating or at least minimizing structural vibration, or noise as it is commonly called, in mechanical components. Such developments range from rotating machine components such as those in turbine engines to sensitive electronic components. A solution to such vibration problems is often achieved with the use of carefully placed discrete vibration dampers.
A common class of discrete vibration dampers serve to dissipate vibrational energy by transforming kinetic vibration energy into thermal energy, which in turn is harmlessly dissipated into the surroundings. Some such devices reduce the transmission of noise by utilizing a shearing action of a highly viscous fluid. Viable fluid based damper designs employ pressure differentials to force the damper fluid through a confined area where the viscous shearing takes place (such as through an orifice). These designs are effective at damping relatively large amplitudes of vibration, but are less effective and oftentimes wholly ineffective with extremely low amplitude vibrations such as those associated with stringent vibration specifications. In the event of small vibration amplitudes, such existing designs are unable to force the damping fluid through the internal energy dissipation paths because they are unable to produce the required internal pressure differentials. Discrete dampers of this type often require several visco-elastic seals to contain the fluid within the damper fixture. Consequently, in the event of extremely low vibration amplitudes, the visco-elastic seals yield before sufficient internal pressures can be developed to initiate viscous shearing action.
The present invention was developed in accordance with certain specific requirements discussed below and by meeting these requirements, problems associated with prior techniques have been overcome.
One requirement is that the damper be capable of successful operation in the event that the two structures or components connected by the damper have long term relative displacement or creep between them. This essentially requires the damper to be capable of repositioning its inner damping mechanism during long term (months and possibly years) fluctuations in the relative distance between the two components or structures.
A second requirement is that the damper assembly dissipate not only medium-to-high amplitude vibrations, but also and especially extremely low amplitude vibrations.
A third requirement is that the damper be capable of maintaining its structural integrity during a dynamic shock event, for example of the type experienced by naval shipboard equipment. Such an event could invoke excessive impulse forces on the damper. Existing damper designs, in addition to their inability to satisfactorily dissipate low amplitude vibration, are not likely to survive large impulsive forces characteristic of dynamic shock.
The present invention provides a damper assembly that meets all of the above requirements. Specifically, the damper assembly of this invention is a tunable design, capable of dissipating high but especially extremely low amplitude vibration while also having both the ability to reposition itself in the event of long term relative displacement, between connected structures, and to withstand dynamic shock.
In an exemplary embodiment of the invention, the damper assembly of this invention is comprised of three components: (1) a tuned visco-elastic spacer; (2) a sealed piston/cylinder assembly; and (3) a viscous damping mechanism.
More specifically, the damping assembly according to this invention includes a first attachment element or bracket for securing the damping mechanism to a structure, the vibration of which is to be damped. A tuned visco-elastic spacer block is secured to the element or bracket at one end, and to a cylinder of a piston/cylinder assembly at its other end. A piston is slidably received in the cylinder with a predetermined clearance between the peripheral surface of the piston and the cylinder wall. A fluid line extends from the cylinder to connect cylinder chambers above and below the piston. This fluid line is also provided with a pressure relief valve for a purpose described further herein. A piston rod extending from the piston and through the opposite end of the cylinder is received within a housing of a viscous damping device, the piston rod having a plurality of discs mounted thereon in a predetermined, spaced relationship. An interior wall of the housing is also formed with a plurality of disc-like surfaces projecting toward the radial center of the housing, with aligned apertures therein to accommodate the piston rod.
The other or remote end of the viscous damping housing is secured to a second attachment element or bracket by which the damper is secured to a stationary supporting (relative to the vibrating structure) surface or substrate.
The arrangement of piston rod discs, and housing discs, is such that the respective groups of discs are interdigitated, with a close axial spacing between adjacent piston and housing discs. It is well understood that relative axial movement between the groups of discs causes the viscous fluid to be compressed between the discs, resulting in a high velocity viscous shearing of the fluid, and hence energy dissipation through the generation of heat.
The above described elements or components of the damper are thus connected in series and dissipate energy as described below. For the occurrence of long term creep and/or extremely low frequency vibrations, the piston and cylinder move relative to each other with minimal force transmitted to the lower portions of the structure, i.e., to the viscous damping mechanism. Thus, equilibrium is substantially maintained between the fluid in the upper and lower chambers of the piston/cylinder assembly because of the slow relative motion of the cylinder relative to the piston permitted by reason of the predetermined clearance between the piston and the cylinder side wall. Springs located in upper and lower portions of the viscous damping housing tend to bring the discs back to their original equilibrium position to insure consistent damping performance.
For low (and high) frequency vibration, the piston/cylinder side wall clearance is sufficiently small to restrict increased fluid velocity within the clearance. Thus, alternating high and low pressures will be developed in the upper and lower chambers of the cylinder, thereby transferring force between the piston and the surrounding cylinder, so that they act as a substantially rigid unit. This force transmission will ultimately result in axial movement of the piston rod and consequent fluid shearing action in the viscous damping mechanism below the piston/cylinder assembly to thereby dissipate the kinetic vibration energy.
The dynamic shock requirements of the damper are addressed by incorporating the relief valve in the fluid line arrangement into the cylinder walls on either side of the piston. For a severe shock event, large pressures would develop in either the upper or lower piston chamber. The valve within the line located in the side of the cylinder would be adjusted to open at a certain predetermined pressure, thus allowing rapid fluid flow between the upper and lower chambers of the piston/cylinder assembly. This would eliminate any force build-up within the damper which could otherwise adversely effect the damping mechanism.
The above described assembly is tunable in two ways. First, the viscous damping device can be designed to deliver a level of damping dictated by the specific application. This may be achieved by providing appropriate spacing between the discs of the damping device, since this controls the level of damping.
Second, the assembly as a whole may be designed to have a certain natural frequency which eliminates the possibility of a "short" for high frequency vibration between the two structures interconnected by the damper assembly. The natural frequency of the assembly can be varied by careful selection and design of the visco-elastic spacer. In other words, it is possible to match the stiffness of the spacer with the mass of the piston/cylinder and viscous damping device to create the natural frequency in accordance with specific applications so that energy at frequencies above the natural frequency is not transmitted through the damper assembly. Thus, the assembly has the characteristics of a low pass filter.
The device does not damp extremely low frequency vibrations, which are, in fact, accommodated as long term creep by the assembly. The device also does not dissipate high frequency vibrations, i.e., vibrations at a frequency higher than the natural frequency of the assembly as discussed previously.
Thus, in accordance with one exemplary embodiment of the invention, therefore, there is provided a tunable vibration damper assembly comprising (a) a visco-elastic spacer; (b) a piston and cylinder assembly including a piston rod attached at one end to the piston and having a free end extending out of one end of the cylinder, wherein the visco-elastic spacer is attached to the other end of said cylinder; and (c) a viscous damping device operatively attached to the end of the piston rod.
In another aspect, the invention provides a vibration damper assembly adapted to dissipate kinetic energy from at least extremely low amplitude vibrations comprising: an elastic spacer connected at one end to a structure whose vibrations are to be damped, and at the other end to a cylinder of a piston/cylinder assembly; the cylinder enclosing a piston therein defining variable volume chambers in said cylinder above and below said piston; a piston rod extending from said piston, through said cylinder and into a viscous damper housing, wherein said piston rod and an interior surface of said viscous damper housing are provided with interdigitated damping means.
Other objects and advantages of the subject invention will become apparent from the detailed description which follows.